We studied the in vivo recovery, lifespan, and hemostatic effectiveness of CPDA-1 platelet concentrates stored for up to 72 hours at 4 C. A total of 120 CPDA-1 concentrates containing an average (+/- 1 S.D.) of 6.6 +/- 2.0 x 10(10) platelets were prepared. The pH of the units following storage at 4 C was 6.8 +/- 0.2; no unit had a pH below 6.3. Autologous transfusion of six normal volunteers showed that platelets stored at 4 C for 72 hours had an in vivo recovery of 40 +/- 18 per cent and a lifespan of 5.1 +/- 1.5 days. The hemostatic effectiveness of CPDA-1 platelets was determined by platelet counts and template bleeding time measurements in 10 thrombocytopenic patients. Patients receiving 48-hour-stored platelets had a four- to six-hour posttranfusion corrected platelet increment averaging 15,300 +/- 3,200/microliter which was 67 +/- 34 per cent of expected recovery. Four of the five patients transfused with this preparation showed an improved bleeding time. In contrast, three patients receiving 72-hour-stored platelets had a four- to six-hour posttransfusion increment of 5,800 +/- 2,400/microliter that was only 26 +/- 13 per cent of the expected recovery; furthermore, only one of these patients showed any correction of the bleeding time. These data indicate that CPDA-1 platelets are hemostatically effective when stored at 4 C for up to 48 hours.